Non-functional ubiquitin C-terminal hydrolase L1 drives podocyte injury through impairing proteasomes in autoimmune glomerulonephritis

Abstract

Little is known about the mechanistic significance of the ubiquitin proteasome system (UPS) in a kidney autoimmune environment. In membranous nephropathy (MN), autoantibodies target podocytes of the glomerular filter resulting in proteinuria. Converging biochemical, structural, mouse pathomechanistic, and clinical information we report that the deubiquitinase Ubiquitin C-terminal hydrolase L1 (UCH-L1) is induced by oxidative stress in podocytes and is directly involved in proteasome substrate accumulation. Mechanistically, this toxic gain-of-function is mediated by non-functional UCH-L1, which interacts with and thereby impairs proteasomes. In experimental MN, UCH-L1 becomes non-functional and MN patients with poor outcome exhibit autoantibodies with preferential reactivity to non-functional UCH-L1. Podocyte-specific deletion of UCH-L1 protects from experimental MN, whereas overexpression of non-functional UCH-L1 impairs podocyte proteostasis and drives injury in mice. In conclusion, the UPS is pathomechanistically linked to podocyte disease by aberrant proteasomal interactions of non-functional UCH-L1.

The Proteasome Modulates Endocytosis in a Glomerular Cell Type Specific Modality to Secure Kidney Filtration

“…Proteasome substrates prominently accumulated in podocytes of proteasome-inhibited mice as expected 43,49 , however, not in MCs and GEnCs showing that podocytes cannot balance their proteome in the setting of proteasome dysfunction. It is increasingly getting acknowledged that the autophagosomelysosome and proteasome system interact at multiple levels to balance the cellular proteome 12,13 .…”

“…As such, the involvement of the immunoproteasome in the preservation of cellular proteostasis was initially demonstrated in broblasts in the setting of oxidative stress 42 . In line, the de novo expression of the immunoproteasome in the context of podocyte injury 11 is thought to represent a mechanism to preserve proteostasis 36,43 . Hence, the strong immunoproteasome abundance and activity in GEnCs could signify a prominent physiologic need for the degradation of proteasome substrates.…”

Podocyte exopher-formation as a novel pathomechanism in membranous nephropathy

“…6A panel 1 ). PLA 2 R1 density differed in diagnostic biopsies of a well-characterized retrospective PLA 2 R1 + -MN patient cohort 32 in which 18 patients were stratified to a remission group (R; amelioration of serum creatinine, eGFR, proteinuria, albuminuria, and/or serum albumin) within a mean follow-up period of 42,83 ± 5,52 months, and 17 patients to a non-remission (NR) group, within a mean follow-up period of 43,24 ± 9,2 months. In the remission group, PLA 2 R1 density and sPLA 2 R1-abs were low at diagnosis.…”

“…Panel 1 : Scoring range of the PLA 2 R1 aggregate density algorithm (D = density; arbitrary units); red areas in overlay are shown, orange arrows = urinary space PLA 2 R1 aggregates, white arrowheads subepithelial PLA 2 R1 deposition, P = podocyte nucleus, u = urinary space, c = capillary space. Panel 2: PLA 2 R1 aggregate density was quantified in diagnostic biopsies of a retrospective PLA 2 R1 + -MN patient cohort 32 , assembled based on clinical outcome. N = 18 patients were stratified into the remission group R and N = 17 into the non-remission group NR.…”

“…As such, urinary exopher-release was low in the relapsing THSD7A + -MN patient in the setting of high sTHSD7A-abs, and the histological pattern of scarce urinary space aggregates and dense subepithelial deposition predominated in the non-remitting PLA 2 R1 + -MN patient group. Of note, extensive exopher-formation might also have disease aggravating aspects i) by transfer of toxic protein aggregates to surrounding cells, promoting pathology 44 , ii) by exopher phagocytosis through surrounding cells as a starting point for (cross-)presentation and immune activation leading to secondary autoantibody formation against intracellular proteins 32,45 or to epitope-spreading of primary MN autoantibodies 46 , iii) by initiating lipid biosynthesis 27 , which when dysregulated perpetuates podocyte injury 47 , and iv) by substantial loss of plasma membrane required for exopher-formation, necessitating membrane repair. The flux balance between serum autoantibody titer, exopher-formation and release is certainly crucial for the ultimate outcome in MN.…”